The efficiency and performance of internal combustion engines has been the focus of a great deal of attention over the years. There are many parameters which affect the performance of an engine whether it be two-stroke, four-stroke, petrol or Diesel, of normal reciprocating type or of one of its derivatives such as a Wankel engine.
One feature of all internal combustion engines is the need to draw fuel and air into a combustion chamber and to expel burnt gases after combustion has taken place. Apart from the design of the mechanical components such as valves, pistons and combustion chambers, the shape of the inlet and exhaust manifolds has been shown to be important. This is because the shape affects the pressure difference between the combustion chamber and the inlet or exhaust. It is this pressure difference which forces the air/fuel mixture into the chamber, or the burnt gases out of the chamber. The function of turbo-chargers and superchargers, for example, is to increase the pressure in the inlet. The pressure difference between the combustion chamber and inlet or exhaust varies with time throughout the cycle of the engine, and so the complete time histories of the pressure differences can effect engine performance, and not just their time averages, for example.
The design of exhaust pipes and inlet systems is also constrained by the requirement that they must reduce the sound radiated from the engine to acceptable levels. This requirement is becoming increasingly stringent and is often in conflict with the desire for good performance and efficiency.
The `tuning` of exhaust pipes, especially for two stroke engines, is a way of influencing the unsteady pressure in the exhaust but, since the tuning is achieved by the fixed exhaust geometry, the tuning cannot be effective at all engine speeds.
Over the last twenty years there has been a development of active control systems which can be applied to reduce the sound radiated from the end of exhaust pipes. These control systems work by reflecting the sound wave in the pipe back towards the engine. They allow shorter and more direct pipes to be used and so affect the efficiency of the engine under some operating conditions, however they can induce strong acoustic resonances in the exhaust pipe which can lead to increased noise radiation from the walls of the pipe.
The use of two actuators to absorb the sound in a pipe rather than to reflect it is known. However, the technique uses a single channel control system and relies on using actuators with a matched response. It is not suitable for practical applications where the response of the individual actuators can vary over a period of time.
There are other machines which have inlet and outlet systems and whose performance is affected by the dynamic pressures in those systems. Examples include gas compressors (which include screw, tooth, reciprocating and centrifugal types), pumps, turbo-machines etc. The performance of these can be measured in different ways (such as mass flow rate, power output, efficiency etc) and, as with internal combustion engines, there is often a conflicting requirement for noise reduction.